Instructables user dbc1218 has posted a tutorial on creating the USB Bike Generator, which places a small generator onto a bicycle to charge USB devices while pedaling. The invention uses a stepper motor to generate the electricity, and a voltage regulator to maintain a constant 5 volts required for USB ports.  The inventor states that after having two prior iterations of the USB Bike Generator to flatten out some kinks, he discovered a 7:1 ratio for bicycle diameter to generator diameter optimal to balance power and efficiency. The project harnesses human-powered energy to make an eco-friendly mode of transportation even more green.

USB Bike Generator

[via lifehacker]